electric motorcycle
The shuangye mid-drive has the biggest motor you can buy in a factory turn-key ebike right now. It is labeled as a 1,000W drive, but…it is capable of MUCH more. Not only that, it has ISIS cranks and torque-sensing, so…let’s get started.
bafang mid drive
Bafangs internal labeling system calls this the MM G510.1000, and it’s design makes several improvements over my favorite drive, the BBSHD. The BBSHD is a kit that slides into just about any frame you like, but the Ultra Max requires a proprietary shell to mount it (see below).
The first thing that pops out at the casual observer is that the Ultra has a larger diameter motor. This increases the amount of leverage that the magnets exert onto spinning the rotor, without any additional watts applied to it, compared to the same watts being applied to a smaller diameter motor with the same copper mass. The other thing that this helps is the efficiency, since the “tangential magnet speed” is faster for a given RPM.
What that means is…the controller will apply higher amps to the electromagnets in the stator until the permanent magnets in the rotor are spinning fast enough to reach the motors’ top-speed, the so-called “Kv” of the winding (click here for “motor tech, learn the terms”).
The faster the magnets pass by each other, the shorter the pulses of watts are…that are applied to the electromagnets. Using lots of small pulses ‘can’ provide the same total power that is applied, compared to using fewer long pulses, but…using long “on” pulses will heat up the MOSFET’s in the controller, and also the electromagnets in the stator.
Be aware the Ultra Max stator is narrower than the BBSHD, but the diameter is larger enough that it still does have more copper mass.
Another thing that is worth noting is that the BBS02 in the picture above uses what’s called “Surface Permanent Magnets” / SPM on the rotor, and the Ultra (along with the BBSHD) uses a style that inserts the magnets a small distance away from the surface of the rotor. This style is being seen more often these days, and it’s called an “Interior Permanent Magnet” motor / IPM.
This design allows the magnets to run cooler, which is important because one of the limitations on how many amps a motor can use is the heat that is generated by “eddy currents”. The stator core is made from a stack of very thin steel plates in order to reduce eddy currents, which are generated anytime a ferrous metal is rapidly passed through a magnetic field.
Using a stator-core that is made from thin laminated plates (coated with a lacquer to electrically isolate one plate from the other) is a cost-effective way to accomplish a limitation of any eddy current heat, but…unlike the laminated stator-core, the magnets are solid chunks of metal. With the older SPM motor designs, the magnet body itself becomes a source of waste heat.
With an IPM, the permanent magnets will “magnetize” the slim section of steel between them and the electromagnets in the stator. This keeps the magnetic field strength in the air-gap at an acceptable level, while still placing the actual permanent magnets a short distance away from the air-gap. Permanent magnets can lose their magnetic power if they get too hot, so…by doing this, you can use more “temporary peak” amps without overheating the magnets.
ISIS cranks, not square taper
The very common “square taper” shank on bicycle spindles is appropriate for the BBS02. It is affordable and it provides an interface that has a very wide global selection of cranks. For instance, if you are looking for an obscure offset in a pair of crank-arms, the square taper spindles (frequently found on street bicycles) will have the best selection…including cranks made of titanium, carbon fiber, aluminum, and steel.
The BBSHD also uses a square tapered spindle, which I believe was simply chosen to save on costs. When comparing the BBS02 to the BBSHD, it was obvious to me (years ago) that the larger BBSHD would be the drive of choice for off-roaders (off-road is where there are no power limits in many countries).
The one place where you can consistently find the stronger (and more expensive) 10-spline “ISIS” format for the spindles is on serious off-road bicycles. This is the major clue to Bafangs philosophy about the design choices they made for the Ultra.
Torque-Sensing
I don’t have a snappy picture to illustrate Torque-Sensing, but it should only take a few paragraphs to explain. Most “Pedal Assist Sensor-PAS” designs allow the bike to add power by simply sensing when you are pedaling (no hand throttle). In fact, some countries in Europe only allow PAS.
The affordable PAS solution is to have a “speed sensor”, where pedaling causes a disc with several embedded magnets to pass by some type of fixed magnetic sensor. However, it can take a 1/4 turn of the pedals to engage, and a half-second of stopped pedals to dis-engage. Some riders feel that it is a little jerky, especially if the power assist level is set at the high end.
A torque-sensor is more expensive, but all of the modern versions that are readily available provide an “instant-on / instant-off” experience when you start and stop pedaling. Most of them even sense the amount of pedal pressure that the rider is applying, and then it adds motor power accordingly. Who likes this? Off-roaders.
Well, actually…everyone likes it, but…off-roaders are consistently willing to pay more for it. It makes sense. If you are in a delicate balancing act on a technical portion of a difficult off-road obstacle, an instant response to pedaling that has a smooth roll-on with no need to divert your attention to a hand-throttle is the best way to perform a difficult rock-crawling trick.
Controller
The pic below shows that the factory controller for the shuangye uses 12 MOSFET’s. A Field Effect Transistor / FET is an “on-off” switch that controls the amps that are fed to the three phases of the motor.
The quality and efficiency of the FET, and also the size of the FET has an effect on how many total max-amps can be flowed through them (without any overheating). Even so, 12 FETs is a LOT! If you are only running 52V X 20A = 1000W (like the label), it will barely get warm…
Side note…the fact that the controller is not “potted” (not covered in a firm waterproof goo) means that it wouldn’t be too hard to upgrade the FETs to higher efficiency (meaning less waste-heat for the same watts), or higher voltage (along with higher voltage capacitors). That being said, it would be easier to splice-in an external controller to use 20S / 72V, if you wanted to experiment…
Gear Reduction
The motor Kv is 55-RPM’s per volt, and the triple-reduction between the motor and cranks is 18:1. Additional gearing can be used between the cranks and the rear wheel.
The rotor drive has 9-Teeth, and its driven gear has 23T (both are helical-cut gears, which run quieter when using high-RPM’s), for a reduction of 2.5:1. The second helical gearset is 66T/25T for 2.6:1 . The third gearset is 46T/17T for 2.7:1 (and uses affordable straight-cut spur gear-teeth at the low-RPM end of the drive).
The Proprietary Interface
The benefit of a conventional mid-drive kit is that it can slide into the “bottom bracket” of almost any bicycle frame that you like. The drawback is that it limits the arrangement and sizes of various components inside the drive. Proprietary drives have much more design freedom.
The most popular factory mid-drives are the Bosch, Yamaha, Panasonic, Continental, and others.
What ebikes have it?
The Bafang was first shown at the Taipei bicycle show in 2017, and it was marketed as being more powerful and less expensive than the popular Bosch mid drive.
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